In an equivalent time sampling oscilloscope, a phase reference module is used to compensate for jitter in the time base of the oscilloscope. Two signals are measured—a data signal and a clock signal. The clock signal is a sinusoid or a similar sine-like signal that is aligned to the data signal. The data signal, on the other hand, can be almost any other type of signal. Traditionally, timing errors are calculated by the system shown in FIG. 1. The timing correction is made to the data signal by measuring the phase of the clock signal and using the clock signal phase measurement to adjust the timing of the data signal based on the calculated error.
FIG. 1 depicts a traditional timing correction system traditionally used in time sampling oscilloscopes. A clock signal 102 synchronized to the data is input into the system 100 through the clock signal input 100. The clock signal 102 is split into two clock signals 104 and 106 via a splitter 120.
Each clock signal 104 and 106 is sent to a respective sampler 108 and 110. Samplers 108 and 110 sample the clock signal at different phases that are ideally 90 degrees apart. That is, the samplers 108 and 110 are strobed by a strobe 122 at 90 degrees offset from each other. The samplers are set to strobe at times that are separated by 90 degrees so that the in-phase and quadrature components of the clock signal can be measured. When the phase between the two samples taken by the samplers 108 and 110 are exactly 90 degrees apart, the error made in estimating the phase of the clock signal is a function of only the amplitude noise of the clock and noise present in the samplers 108 and 110.
The output signals from the samplers 108 and 110 are each input into respective analog-to-digital converters (ADC) 112 and 114. The digitized signals are then sent to the phase measurement unit 116 so the phases of the digitized signals can be calculated. Once the phases have been calculated, a timing correction can be calculated in a processor 118. The in-phase and quadrature components of the digitized signal are used to estimate the phase of the clock, and from the phase of the clock the timing error of the sampling test and measurement instrument can be estimated.
Because the actual difference between the two samples is not exactly 90 degrees, the jitter measured by the scope becomes a function of the phase of the signal. This leads to non-repeatable results where different values or jitter are measured depending on the phase difference between the clock and a measured data signal by the oscilloscope. This causes different parts of the measured data signal to have potentially different amounts of jitter depending on the phase of the clock at the point of the measurement. This is non-ideal and can lead to user confusion.